A good friend of mine, hey Bill, gave me lots of these cedar strips. They are unified in thickness, but do vary a little in length and width. You need about 60 pieces.
Stake out an area that is 4' x 4'. This will be the size of your box.
Start by laying out your pieces and having them overlap slightly. I used wood glue then nailed them together. I was able to use three pieces to make one 4' side. It might be different for your wood.
Continue working in that pattern around your square. It will be flimsy at first, but as you build more layers it does become much more solid.
The box is taking shape.
I went with 3 layers of strips. I then added a layer of strips that ran all the way around the top to make it look cleaner and give it stability. Adding a similar layer to the bottom would help too.
Here are four finished boxes. They are approx. 7 inches deep. It would require 9.3 cubic feet of soil to fill each box.
I bought inexpensive landscaping fabric to line the bottom and sides of the boxes. This will help keep the weeds out and soil in.
I used staples to attach the fabric to the frame. Make sure you don't staple it too tight. You want to leave enough slack for when you fill it in.
I lined the entire box. With two strips I had enough for some overlap in the middle. I left enough slack at the ends and sides so that I could fold it back once I started to fill in the soil.
Start filling in the box with your soil. Make sure the fabric doesn't tear way from the sides.
As you are filling your box makes sure you fold back the excess fabric and 'bury' it with soil. It makes it look cleaner. When the box is full use a rake to level out the soil.
Measure your sides and mark off the locations for your dividers. I went every 11-12". You need to place 3 dividers going each way to form your 4x4 grid.
My three dividers are placed going one way. I used twine held in place by staples. You could choose another means if you wanted. These are to help keep your garden organized.
The grid is complete. You now have 16 square foot garden plots.
Time to plant your seeds. Have fun.
I would avoid pressure treated wood. The chemicals might leach into the soil. The cedar I used came from a scrap pile of a local furniture producer. Check them out at www.thebearchair.com